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Trouble Shooter

A large number of DTCs are assigned to the throttle actuator control system. But even when a DTC has been stored and the recommended test(s) carried out, this still may not be enough to produce a definitive fault diagnosis.

Stuck Throttle

I received several responses to last month’s column on electronic throttle control. I’d like to devote this month’s column to information provided by Chris Hopkins from Metuchen, NJ. Chris has successfully used an amp clamp and a digital storage oscilloscope to identify a failing actuator motor inside a throttle actuator control (TAC) system. Here, he describes his test methods, and their results.

As a diagnostic tech, there are a few things I really don’t like. One is intermittent faults, and another is diagnosis by substitution. Yet the two often go hand in hand. If you can’t find a fault, you can’t show/prove what the cause is. So you can either mark the repair order as “no trouble found,” or try replacing the “most likely cause.” My thought is that if you change nothing on a vehicle, then there is no reason for the symptom not to return—eventually. That’s why I really want to find a “smoking gun” if I can.

Now it might be an educated guess that results in replacing a part, but it’s still a guess. That seems to be the case too often on parts such as electronic throttle body assemblies. Most are fairly simple—a reversible motor with dual TPS sensors. Yet I seldom have had any luck capturing proof of a glitch, be it TPS or motor codes. So I have replaced the TB units (they are a high-wear item) and sent them on their way. So far all have been no further trouble.

A few weeks back, a 2010 Mercury Milan 3.0L came in with an illuminated MIL and a complaint of intermittent no throttle response. The engine would idle only until it was restarted. The PCM had a DTC P2111 (throttle actuator stuck open) in memory. This is the same DTC experienced by the reader in your previous column on an older Milan with the same system, and it indicates that “the throttle plate is at a greater angle than commanded.” Possible causes include a binding throttle body (stuck open), cross-wired TAC motor circuits, shorted together TAC motor harness circuits or a damaged PCM.

Of course, the system wouldn’t act up for me. The DTC flow chart instructed me to perform a bunch of continuity tests that might have helped if there had been a hard fault. Everything passed during my testing. Pin drag tests were okay. Since a couple tests were for the throttle position sensors, I also swept them on my scope. Both looked fine. The end of the diagnostic flow chart instructed me to replace the TB assembly, but I hadn’t found the smoking gun.

I thought about the code description and tests. This wasn’t a TPS fault code; it was a motor circuit code. I reasoned that the PCM could be detecting only a high- or low-current condition. I attached a low-amp inductive current probe to an actuator motor wire and the scope captured current spikes that varied and inverted as the PCM supplied pulse width modulated (PWM) signals to open/hold/close the throttle blade.

Many of the waveforms I captured looked pretty much like Fig. 1, and they didn’t give a clear indication of a problem. Looking closer, I found a couple spots where the spikes were cut short and looked ragged. Something was bad, but I wasn’t 100% sure what. The motor? A PCM driver? It didn’t look quite right for a wiring fault, but I didn’t have time right then to look at the PCM to motor voltage patterns on the scope.

I wanted to test the throttle body motor without the PCM and wiring, so it occurred to me to treat it like any other motor. I did this by feeding it power and ground, then looking at the current waveform on the scope. I hooked test leads to the TB connector, but fed the applied voltage through a #912 bulb. This allowed enough current flow to open the throttle blade, but limited it to about 1 amp when it reached full travel to prevent damage to the motor or geartrain. The waveform shown in Fig. 2 traces the repetitive pattern of a bad commutator bar in the motor very clearly.

I’ve found this to be an easy test to check the health of the PCM-controlled TB motor. Like fuel pump current waveforms, this isn’t a 100% yes or no test. But if it looks really bad, then you know (and can show) that the part does need to be replaced.